First, the failure caused by capacitor damage is the highest in electronic equipment, especially in the electrolytic capacitor damage is the most common.
Capacitance damage is shown as: capacity decreases; Total loss of capacity; The leakage; Short circuit.
Capacitors play different roles in the circuit and the faults caused by them have their own characteristics. In industrial control circuit board, digital circuit accounts for the majority, capacitor is used as power filter, used as signal coupling and oscillation circuit capacitor is less. If the electrolytic capacitor used in the switching power supply is damaged, the switching power supply may not vibrate and there is no voltage output; Or the output voltage filtering is not good, the circuit due to voltage instability and logic chaos, the performance of the machine is good or bad or can not open the machine, if the capacitor and in the digital circuit between the positive and negative power, fault performance is the same as above.
This is particularly obvious on the computer motherboard, many computers have been used for a few years, sometimes can not open the machine, sometimes can start the phenomenon, open the case, often can see the phenomenon of electrolytic capacitor bulge, if the capacitor removed to measure the capacity, found that the actual value is much lower.
The lifetime of capacitors is directly related to the ambient temperature. The higher the ambient temperature is, the shorter the lifetime of capacitors will be. This rule applies not only to electrolytic capacitors but also to other capacitors. Therefore, when looking for fault capacitors, we should focus on the capacitors close to the heat source, such as those near the heat sink and high-power components. The closer they are, the greater the possibility of damage.
The power of the once took an X-ray flaw detector, users reflect the smoke came out from the power supply, open the case after arrival a 1000 uF / 350 v large capacitor has the oily stuff out, tear open come down to a capacity of only a few tens of uF, also found that only the capacitance of the rectifier bridge and heat sink nearest, other from far is intact, the capacity is normal. In addition, there is a short circuit in the ceramic capacitor, it is also found that the capacitor is close to the heating parts. So in the overhaul of the search should be focused.
Some capacitors are so leaky that they can become hot when touched with a finger. They must be replaced.
In the maintenance of the ups and downs of the fault, excluding the possibility of bad contact, most of the fault is caused by capacitor damage. So in the event of such a failure, you can check the capacitor, replacing the capacitor is often surprising (of course, also pay attention to the quality of the capacitor, to choose a better brand, such as rubies, black gold, etc.).
Characteristics and discrimination of resistance damage
Often see a lot of beginners in the maintenance circuit on the resistance of torture, it is dismantled and welding, in fact, repair much, as long as you understand the damage characteristics of the resistance, there is no need to go to great trouble.
Resistance is the most numerous component in electrical equipment, but not the one with the highest damage rate. Resistance damage to open circuit is the most common, the value of the resistance to increase is rare, small value of the resistance is very rare. There are common carbon film resistance, metal film resistance, wire winding resistance and insurance resistance.
The most widely before two kinds of resistance, the characteristic of its damage is a low resistance (below 100 Ω) and high value (more than 100 k Ω) damage rate is higher, intermediate resistance (such as a few hundred to tens of thousands of European) little damage; The second is the low resistance value of the damage of the resistance is often burned black, it is easy to find, and the high resistance value of the damage of the resistance is rarely traces.
Wire-wound resistance is usually used as a large current limiting, the resistance value is not large. Cylindrical line around the resistance when burned out, some will be black or surface skin, crack, some have no trace. Cement resistors are wire-wound resistances that may break when burned, otherwise there is no visible trace. When the safety resistor burns out, some of the surface will blow off a piece of skin, some have no trace, but never burned black. According to the above characteristics, when checking the resistance, we can put some emphasis on it and find out the damaged resistance quickly.
According to the characteristics of the listed above, we can first look at the low resistance values for any signs of blackened on printed circuit boards, and then according to the damage to the vast majority of open or value become larger and easy to damage the characteristics of high resistance values, we can use the multimeter direct volume very high resistances in the circuit board at both ends of the resistance of the resistance, if the amount of resistance is greater than nominal value, is the resistance must be damaged (pay attention to, such as stable resistance showed only after to conclusions, because may shunt capacitance in circuit components, there is a charge and discharge process), if the amount of resistance is smaller than nominal value, you generally don’t have to ignore it. So on the circuit board every resistance is measured, even if “wrong kill” a thousand, will not miss one.
The discriminating method of operational amplifier
The discriminant of operation amplifier stand or fall has certain difficulty to quite a lot of electron to maintain person, not only the relation of educational level (there are a lot of undergraduate students at hand, if do not teach, affirmation won’t, taught to also want for a long time just comprehend, still have a graduate student that learns frequency conversion control with the adviser technically, also be such unexpectedly! , discuss together with everybody here, hope to be helpful to everybody.
Ideal operational amplifiers have the characteristics of “virtual short” and “virtual break”, which are very useful for analyzing operational amplifiers used in linear applications. To ensure linear operation, the operational amplifier must operate under closed loop (negative feedback). If there is no negative feedback, the op amp under open loop amplification becomes a comparator. If you want to judge the quality of the device, it should be clear whether the device is used as an amplifier or a comparator in the circuit.
No matter what type of amplifier, there is a feedback resistor Rf, then when we repair can check the feedback resistance on the road from electricity, using a multimeter to check output and reverse the input, the resistance between the if large, such as more than a few M Ω, then we can be sure the device is probably do more for, if the value 0 Ω to dozens of smaller k Ω, is to check whether the resistance between output and reverse the input, amplifier using words will do.
According to the principle of virtual amplifier short, that is to say, if the operational amplifier working properly, the same input and reverse voltage of the input will be equal, even though there are differences also mv level, of course, in some of the high input impedance circuit multimeters resistance will be a little influence on voltage test, but generally not more than 0.2 V, if the difference of more than 0.5 V, the amplifier will no doubt bad! (I used the FLUKE179 multimeter)
If the device is used as a comparator, the same input and the opposite input are allowed to be different.
If the voltage is detected not conforming to this rule, the device will be broken!
This way you can tell whether an op amp is good or bad without using substitution and removing the chip from the circuit board.
A tip for testing SMT components with multimeters
Some patch components are very small, it is not convenient to test and repair with ordinary multimeter pen, one is easy to cause short circuit, the other is the insulation coating of the circuit board is not easy to contact the metal part of the element pin. Here is a simple method, will bring a lot of convenience to the detection.
Take two of the smallest sewing needles, (depth of industrial control maintenance technology column) to the multimeter pen close, and then take a multi-strand cable in the thin copper wire, with thin copper wire to watch pen and sewing needle tied together, and then use soldering. This way, the SMT components are no longer short-circuited when measured with a stylus with a tiny tip, and the tip can Pierce the insulation coating and hit key parts without the bother of scraping the film.
Fifth, the circuit board public power short circuit fault repair method
Circuit board maintenance, if public power short circuit fault often head is big, because many devices are sharing the same power supply, each device in the switching power supply, there is the suspicion of short circuit, if the plate element is not much, adopt the way of “hoe the earth” will short-circuit point can be found, if too many components, could “hoe the earth” hoe to situation will depend on luck. Recommend a more effective method here, use this method, get twice the result with half the effort, often can find fault quickly.
To have a voltage and current adjustable power supply, voltage 0-30v, current 0-3a, this power supply is not expensive, about 300 yuan. Will open circuit voltage to the device power supply voltage level, current to minimum first, add the voltage in the circuit of the power supply voltage 5 v points such as the 74 series chips, and 0 v side, depending on the degree of short circuit, the current increases slowly, with the hand touch device, when touch to a certain device heating obviously, this is often damaged components, can be removed to further confirm the measurement. Of course, the operating voltage must not exceed the operating voltage of the device, and can not be reversed, otherwise it will burn out other good devices.
Six, a small rubber, solve big problems
More and more used in industrial control boards, many ways of board with gold finger is inserted into the slot. Because of the bad industrial environment, dust, humidity, corrosive gas environment produces poor contact failure is easy to make the card, a lot of friends may be the way to solve the problem by changing the interface card, but the cost of buying the card is very considerable, especially of some of the imported equipment of the card. In fact, we might as well use an eraser on the gold finger repeatedly rub a few times, clean up the dirt on the gold finger, try again, maybe solve the problem! The method is simple and practical.
Vii. Analysis of intermittent electrical faults
Various electrical faults may include the following situations in terms of probability:
1. Poor contact
Bad contact between the board card and the slot, broken cable, bad contact between the plug and terminal, virtual welding of components, etc.
For digital circuits, under specific conditions, the fault will appear, it may be true that the interference is too large to affect the control system to make it wrong, there are also circuit board individual component parameters or overall performance parameters change, so that anti-interference ability tends to the critical point, so that the fault;
3. Poor thermal stability of components
From a large number of maintenance practices, the first is the electrolytic capacitor thermal stability is not good, followed by other capacitors, transistors, diodes, IC, resistance, etc.
4. Moisture and dust on the circuit board
Moisture and dust will conduct electricity, with resistance effect, and in the process of thermal expansion and cold contraction of the resistance value will change, this resistance value will be in parallel with other components, this effect is strong will change the circuit parameters, so that the fault occurs;
5. Software is also a consideration
Many parameters in the circuit are adjusted by software, and the margin of some parameters is adjusted too low and in the critical range. When the operating condition of the machine conforms to the reason of software fault determination, alarm will appear.